1. Field of the Invention
The present invention relates to lighting control of a lamp of an image projector.
2. Description of the Related Art
An image projector, which reproduces an image with using an image data outputted from an external apparatus such as a personal computer or a video camera and projects the image on a screen by a projection lens, is conventionally known.
Since the image projector enlarges the image on the screen disposed at a long distance, it uses a high intensity discharge lamp as a light source for displaying the image brightly. In the discharge lamp, it is known that a load between terminals of the lamp where the discharge occurs (hereinafter abbreviated as load of the lamp) varies after starting the lighting of the lamp, so that a driving voltage necessary for lighting the lamp (hereinafter abbreviated as lamp voltage) varies correspondingly. Specifically, the lamp voltage is relatively low just after the start of the lighting, and the lamp voltage gradually increases to be stable. Furthermore, the lamp voltage of the discharge lamp entirely becomes higher when life of the discharge lamp reaches to a late stage.
If the driving voltage applied to the discharge lamp is set to be constant regardless of such the behaviors of the discharge lamp, the luminance of the discharge lamp becomes lower corresponding to the increase of the load of the lamp. Thus, it is necessary to execute a feedback control of the driving voltage by a driving circuit of the image projector in a manner so that the luminance of the discharge lamp is maintained substantially constant. In other words, it is necessary that the power consumption of the discharge lamp is maintained substantially constant.
On the other hand, when the driving voltage of the discharge lamp is made too high due to the feedback control, the discharge lamp involves a risk of bursting. Thus, a safety circuit is conventionally provided in the driving circuit of the discharge lamp in a manner so that the safety circuit senses a value of the driving voltage of the discharge lamp, and outputs an alarm or extinguishes the discharge lamp when the value of the driving voltage becomes equal to or larger than a predetermined reference value.
Conventionally, various technologies are proposed for estimating the life of the discharge lamp and announcing a time for replacing the discharge lamp or occurrence of burning out of the discharge lamp.
In a lighting apparatus of a discharge lamp shown in Publication Gazette of Japanese Patent application 5-36481, the above-mentioned behaviors that the lamp voltage is lower just after the start of the lighting, and the lamp voltage gradually increases to be stable, and the lamp voltage of the discharge lamp entirely becomes higher when the life of the discharge lamp reaches to a late stage is utilized for announcing that the discharge lamp should be replaced. The lighting apparatus senses the lamp voltage after passing a predetermined term from the start of the lighting of the discharge lamp, counts a number of the cases that the lamp voltage becomes equal to or higher than a predetermined reference voltage, and displays a message to replace the discharge lamp when the number becomes larger than a predetermined reference number.
In another lighting apparatus shown in Publication Gazette of Japanese Patent application 2000-48989, the safety circuit is activated after passing a predetermined term from the start of the lighting of the discharge lamp in order to prevent malfunction of the safety circuit due to the increase of the lamp voltage in a short term when the life of the discharge lamp reaches to a late stage.
A current, however, flows in the discharge lamp, even when the discharge lamp has not been lighted due to the driving voltage is lower. The conventional driving circuit of the image projector senses the driving voltage applied to the discharge lamp and only judges whether it is in critical condition or not by comparing a value of the driving voltage with the predetermined reference value corresponding to an upper limit of the permissible driving voltage. Thus, it is impossible to judge whether the discharge lamp has been actually lighted or not.
Furthermore, when the feedback control is executed under the condition that the driving voltage is lower, the current flowing in the driving circuit becomes larger. Thus, there is a possibility that the critical condition such as over heat occurs due to the over current.
Still furthermore, when it is attempt to relight the discharge lamp just after the discharge lamp has been extinguished (it is called “hot-start”), the temperature of the discharge lamp has become too high so that the load of the lamp has been too large. Thus, it is necessary to make the driving voltage much higher so as to relight the discharge lamp. Thus, the discharge lamp could be damaged. With respect to such the problem of the hot-start, the same goes for the above-mentioned conventional lighting apparatuses.